Erasable writing medium suitable for use in ball point pens

ABSTRACT

An erasable writing medium suitable for use in ball point pens comprising a discontinuous phase comprising a solid colorant and a homogeneous continuous phase including a matrix material having cohesive properties exceeding its property to adhere to the substratum being written upon, as well as various components for coverting the phases into a writing medium. Upon being dispersed onto the writing surface, the matrix material precipitates from the continuous phase and entraps the colorant thereinto, thus preventing the colorant from penetrating the pores of the writing surface and also preventing the colorant from affixing itself to the solid members of the writing surface. A colored layer is thus formed on the writing surface which is easily removable without damage to the surface.

United States Patent 11 1 Daugherty et a1.

[451 Apr. 1, 1975 1 1 ERASABLE WRITING MEDIUM SUITABLE FOR USE IN BALL POINT PENS [75] Inventors: Phillip M. Daugherty, Monrovia,

Calif; Thomas E. Palmer, Stockbridge; Wilmot D. Tidd, Atlanta, both of Ga.

[73] Assignee: Scripto, Inc., Atlanta, Ga.

[22] Filed: Jan. 23, 1973 [21] Appl. No.: 326,094

Related U.S. Application Data [63] Continuation-impart of Ser. No. 751,759, Aug. 12,

1968, abandoned.

[52] U.S. Cl.. 260/33.2 R, 260/33.6 UA, 260/42.21, 106/32, 106/22, 106/19 [51] Int. Cl. C081 45/28, C08g 51/28 [58] Field of Search 260/33.6 R, 33.6 UA, 41 C, 260/332, 91.1 M, 91.1; 106/22, 32,19

[56] References Cited UNITED STATES PATENTS 2,697,084 12/1954 Eger 260/9l.1 M 2,799,669 7/1957 Zoss 260/91.1 M 2,950,285 8/1960 Miller 106/22 OTHER PUBLlCATlONS Kir lggthmer Encyclopedia of Chemical Technology, p.

Printing Inks Their Chemistry and Technology, Ellis.

Primary E.vaminer-Theodore Morris Attorney, Agent, or FirmNewton, Hopkins & Ormsby [57] ABSTRACT An erasable writing medium suitable for use in ball point pens comprising a discontinuous phase comprising a solid colorant and a homogeneous continuous phase including a matrix material having cohesive 13 Claims, No Drawings ERASABLE WRITING MEDIUM SUITABLE FOR USE IN BALL POINT PENS CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part application of applicants prior copending application Ser. No. 751,759, filed Aug. 12, I968 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains to a truly erasable writing medium that can be satisfactorily dispensed through a ball point writing instrument to give a mark that can be removed with comparative ease with a substantially non-abrasive eraser.

2. Description of the Prior Art Many, frequently interrelated, factors must be taken into account in the design of writing medium. These factors fall into two categories, namely, those which stem from the type writing instrument to be used in dispensing the writing medium onto the writing surface, and those which are concerned with the characteristics which it is desired that the writing medium possess after being so dispensed.

Writing instruments available to the designer of the writing medium include the ball point pen, several variations of the so-called softor porous-type pens, and fountain pens. In some instances, such as is the case with pencils or crayons, the writing medium itself is in effect the dispensing vehicle.

Insofar as the characteristics of the writing medium in the after-dispensed state are concerned, appearance and relative indelibility are the most important. In most cases, the appearance of the writing medium means little more than the color of the written line. Indelibility is related to the ease of removal of the written line. Thus, an indelible writing medium is one which is resistant to removal from the writing surface. Obtainment of a high degree of indelibility is not necessarily desirable. For example, a designer may want a writing medium which is not easily removable by mechanical means from the writing surface, or, on the other hand, a designer may want a writing medium which is easily removable. A writing medium having this latter attribute is generally referred to as an erasable writing medium, and if such a medium is to be considered as being truly erasable, it must be readily removable from the substratum from which it has been applied without any damage of any significant degree to the area of the substratum involved.

In order to understand the nature of this invention, consideration must first be given to the conditions which the designer of an erasable writing medium must avoid. This in turn requires at least an elementary knowledge of the structure of paper, since paper is the substratum most commonly employed as a writing surface.

Paper is essentially a mat of randomly oriented cellulose fibers. Thus, paper consists of solid structural members, namely the cellulose fibers, having numerous minute voids therebetween. From this description, it may readily be seen that the designer of an erasable writing medium must prevent the colorant portion of the medium from penetrating to any substantial degree into the voids in the surface being written upon, because the subsequent removal of the colorant by mechanical means could not be accomplished without damage to the writing surface. Likewise, it may readily be seen that for a writing medium to be erasable, the colorant particles must be prevented from affixing themselves, either by chemical reaction or as a result of mere physical attraction, with any substantial degree of permanence to the solid members of the paper substratum.

In the past, attempts to achieve a truly erasable writing medium for a ball point pen have been notably unsuccessful. These attempts have been employed by two somewhat overlapping approaches to solving this problem.

In one approach, reliance has been placed upon the ability of a superabrasive eraser to remove ordinary ball pen inks from the paper substratum normally used for writing purposes. As discussed above, such substratum is porous to some degree and the ball pen inks in ordinary usage tend to penetrate those pores. In addition, the colorants in such inks tend to affix themselves to the fibers which constitute the solid portion of the substratum. Therefore, the only way to erase markings made with ordinary ball pen inks is to physically remove a substantial number of the fibers in the vicinity of the markings. As a result, the paper substratum is generally so damaged during the erasure process that it is rendered unsuitable for any further use as a writing surface.

In the other approach to an erasable writing medium, attempts were made to modify the ink so as to prevent the colorant portion thereof from penetrating the pores of the paper substratum. These attempts consisted of substituting pigment-type colorants for the dye-type colorants usually used in ball pen inks. The reasoning behind this approach was that since pigment-type colorants normally have a greater particle size than dyetype colorants, the pigment-type colorants would become substantially immobile upon being deposited upon the surface of the paper substratum and, therefore, would not tend to penetrate into the pores of the paper. However, this line of reasoning overlooked the fact that in a ball point writing instrument, the ink is dispensed through a very minute clearance between the ball and socket and that any pigment-type particle which is small enough to be so dispensed will tend to behave as if it was a dye-type particle. Therefore, the colorant portion of such inks was not immobilized to any significant degree and as a consequence, no substantial advantage was gained through substituting the pigment-type colorant for the dye-type colorant insofar as erasability was concerned.

If a writing medium is to be truly erasable, the colorant content therein, after being dispensed onto the written-upon surface, must remain in such a location that it can be readily removed therefrom with a substantially non-abrasive eraser. Further, the colorant must not be allowed to affix itself either through chemical reaction or through ordinary physical attraction with any substantial degree of permanence to the written-upon surface. If these objectives are to be accomplished, the colorant content of the writing medium must be prevented from penetrating the pores of the substratum and must be shielded from intimate contact with the solid members of the writing surface. Further, if such a medium is to be dispensable from a ball point pen, it follows that the medium must possess physical characteristics which do not differ widely from those of ordinary ball pen inks.

SUMMARY OF THE INVENTION The above objectives for an erasable writing medium have been obtained by the present invention which functions as follows. The writing medium as a whole is dispensed from a ball point writing instrument onto the surface of a substratum, generally writing paper. lmmediately prior to being so dispensed, the writing medium is in the form of a heterogeneous mixture having a continuous phase and a discontinuous phase. The discontinuous phase, which appears to be a homogeneous solution, contains all of the remaining materials making up the formulation of the writing medium.

As soon as the writing medium is dispensed onto the surface of the substratum, several successive actions occur. First, one or more of the components of the continuous phase, which had been incorporated therein because of their volatility characteristics, begin to evaporate with considerable rapidity. The loss of these components destroys the apparent homogenity of the continuous phase with the result that one of its components, referred to hereinafter as the matrix material, precipitates therefrom and in so precipitating captures substantially all of the colorant particles, thus forming a colored layer on the surface of the porous substratum. The remaining components of the continuous phase, which are now substantially free of any colorant particles, penetrate into the voids of the porous substratum. These remaining components are included in the-formulation of the writing medium for the purpose ofconverting the mixture of pigment-type colorant, volatile components, and matrix material into a form suitable for use in a ball point pen.

The preferred colorants are copper phthalocyanines; the preferred volatile component is one with an evaporation rate of 3 to on a relative numerical scale on which ethyl ether is assigned an evaporation rate of l, such as toluene; and the preferred matrix material is polyvinyl methyl ether.

DESCRIPTION OF THE PREFERRED EMBODIMENT In this invention, a truly erasable writing medium has been obtained by combining selected materials so as to produce a writing medium which can be successfully dispensed with a ball point pen and which functions as described hereinbelow.

While in the ball point pen prior to being dispensed onto the writing surface, the writing medium is in the form of a heterogeneous mixture consisting of a continuous phase and a discontinuous phase. The discontinuous phase consists of particles of a pigment-type colorant. As used in the description of this invention, pigment-type colorant has the meaning of a material which is substantially insoluble in its immediate environment. The continuous phase comprises a homogeneous mixture of all of the remaining components of the writing medium. As herein used, homogeneous implies that all of these components are mutually soluble. As soon as the writing medium is dispensed onto a typical writing surface, which consists of matted cellulose fibers having numerous, minute voids therebetween, several successive actions occur spontaneously.

First, one or more of the components of the continuous phase being to evaporate with considerable rapidity. The loss of these components changes the solvency characteristics of the remaining components of the continuous phase. As a result, a component of the continuous phase, the matrix material, precipitates therefrom, and in so precipitating captures, for reasons which are not fully understood at this time, substantially all of the colorant particles. The remaining components of the heretofore continuous phase, which are 7 now virtually free of any colorant particles, penetrate into the voids of the substratum.

The colorant particles which are now encapsuled in the matrix material thereby remain'on the surface of the paper.

Thus, in summary, the writing medium which, immediately prior to being dispensed was a relatively stable mixture, separates upon being dispensed onto the porous substratum which serves as the writing surface into three separate parts: one part volatizes and disappears into the atmosphere; the second part penetrates into the porous substratum, and thus for all practical purposes, disappears; and the third part forms a colored layer on the surface of the porous substratum. This colored layer satisfies the previously established criteria for a truly erasable writing medium by virtue of the fact that the colorant and the matrix material mutually interact to prevent any substantial movement of the colorant into the pores of the writing surface and likewise to prevent the colorant from coming into sufficiently intimate contact with the solid members of the substratum to affix itself thereto.

The failure of this mixture of colorant particles and matrix material to penetrate the voids of the paper appears to result from the fact that such a mixture of a solid (the colorant) in a high-viscosity liquid (the matrix material) constitutes what is commonly referred to as dispersion. It is well established that dispersions commonly possess a physical property known as a yield value. The relative concentration of the solid colorant in the dispersion confers upon the dispersion a sufficiently high yield value which serves to stabilize the dispersion and thus tends to inhibit any further movement thereof into the pores of the writing surface. In such a dispersion, each particle of solid matter of colorant is surrounded by a layer of liquid or matrix material; thus, the matrix material forms a barrier between the colorant particle and the porous substratum.

However, immobilization of the colorant on the writing surface would not necessarily insure that the colorant could be readily removed therefrom. Such ease of subsequent removal is insured by selecting as the matrix component a material which is highly cohesive, that is, strongly attracted to itself, and which displays a minimum affinity for the cellulose fibers which constitute the solid members of the writing surface. When this colored layer is rubbed with even a soft, i.e., relatively abrasive-free, eraser, the comparatively weak adhesive forces existing between the matrix material and the substratum material are overcome. As a result, the colored mark (layer) is removed or erased from the writing surface without any substantial damage to the writing surface resulting therefrom. The strongly cohesive nature of the matrix material tends to insure that the colored layer is removed more or less as a whole and, therefore, tends to prevent fragmentation thereof. This serves to assist in the obtainment of a clean erasure of the writing medium from the writing surface. it will be understood that since the colorant particles are firmly embedded in the matrix material, removal of the matrix material also removes substantially all of the colorant portion of the erasable writing medium.

From the preceding, it may be seen that erasability of the writing medium is attained primarily by preventing the colorant therein from reaching locations in the writing surfacewhich would be inaccessible to an ordinary rubber erasure and by'preventing the colorant from affixing itself to the cellulose fibers. The erasability feature of the writing medium is accomplished through the colorant particles in a material which itself is not an adhesive to cellulose.

The functional behavior of the writing medium above is attainable only through a judicious selection of the various individual components of the erasable writing medium. The several factors upon which these selections are based and the material found to best satisfy these requirements are as follows:

1. Functional Components A. Colorant The colorant must have the capability of forming a stable dispersion in the continuous phase of the writing medium for whatever length of time elapses between manufacture of the writing instrument and completion of its normal period of usage by its purchaser. Further, with specific reference to a ball point pen, the size and shape of the individual particles of the colorant must be such that these particles can pass readily between the rotating ball of the pen and the socket within which the ball is enclosed. The dimensions of this clearance between the ball and its enclosing socket are not readily determinable, but it has been estimated that they are on the order of 0.000020 inch.

In addition to the preceeding functional properties, the colorant must possess a shade or hue which is satisfactory to the ultimate user of the writing medium. Also the colorant must be permanent to sunlight, ink eradicators, water and other solvents to whatever degree desired by the designer.

A number of types of pigment colorants were found which would meet the above qualifications, but as a class, the' copper phthalocyanines were generally the best, with CYANBLUE BNF 55-3750, marketed by the American Cyanamid Company, Bound Brook, NJ. 08805, being the preferred colorant.

B. volatizing Component Inasmuch as the primary function of the volatizing agent is to begin to evaporate as soon as the erasable writing medium is dispensed onto the writing surface, first consideration in the selection of such materials was given to their evaporation rates. Such rates are difficult to predict accurately from such physical data as boiling points, vapor pressures, heats of vaporization and the like. Therefore, experimentally-determined data as published by Dr.

Henry Fleming Payne (ORGANIC COATINGS TECH- NOLOGY, Vol. I, published by John Wiley and Sons, New York, 1954 edition, page 4l5) were relied upon. In this data, evaporation rates are expressed on a relative numerical scale in which the chemical compound, ethyl ether, is used as a point of reference; that is, ethyl ether is assigned an evaporation rate of l. Utilizing these data, it was found that the compounds most suitable for use as the volatilization component possessed an evaporation rate in the range 3-]5. In addition, it is necessary that the volatizing compound be a strong solvent for the matrix material and that it be compatible with the remaining components of the writing medium. Our preferred material for this usage is toluol (toluene) 6 which has an evaporation ,-r at e, as published in the Payne data, of 6.1.

C. Matrix Material As has already been mentioned, the primary criterion upon which the selection of the matrix material is based is that its cohesive properties must exceed to a considerable degree any tendency of the material to adhere to the cellulose fibers commonly found in writing papers. Additionally, the matrix material must be readily soluble in the volatizing component of the writing medium but be relatively insoluble in those components which remain after the volatizing component has evaporated. It was further found that viscosity of the matrix material must be comparatively high when compared with many materials commonly employed in writing medium. In many cases, it was found impracticable to make a direct viscosity determination. Therefore, as a matter of convenience, such detemiinations were normally made on a 50% solid solution of the matrix material in toluene. On this basis, the preferred viscosity was found to be 25,000 i 2,000 cps at 77F. Although several materials were found to be suitable for use asa matrix material, the preferred embodiment is a polyvinyl methyl ether sold by the GAP Corporation, West 51st Street, New York, NY. 10020, under the trademark GAN- TREZ M-556.

2. Miscellaneous Components In addition to the components listed above as being functionally necessary in the erasable writing medium of the present invention, other components are required for the purpose of converting the mixture of functional components into a physical form suitable for use in a ball point pen. These miscellaneous components inlcude dispersing agents for the pigment-typecolorants, lubricants, corrosion inhibitors, solvents for use in adjusting the viscosity of the erasable writing medium, and surface active agents for use in adjusting the wetting characteristics of the medium insofar as the, rotatable ball ofthe pen is concerned. In general, these miscellaneous components were selected on an empirical basis with the final criterion in each instance being the ability of the component in question to impart to the remainingcomponents of the erasable writing medium the physical properties desired.

The preferred embodiments of the, various miscellaneous components are:

a. Pigment Dispersing Agent QUADROL, a trademark of the Wyandette Chemicals Corporation, Wyandette Mich. 48192, for N:N:N' :N'tetrakis-(2-hydroxypropyl)-ethylenediamine;

b. Corrosion Inhibitor QUADROL, as previously identified; 1

c. Lubricant any medicinal-grade, medium viscosity, white mineral oil;

d. Viscosity Adjustment Solvent DOWANOL EPH, the trademark for mixed phenyl ethers of ethylene glycol and diethylene glycol, as marketed by the Dow Chemical Company, Midland, Mich. 48640", and

e. Wettability Adjustment Agent SARKOSYL O, the trademark of the Ciba-Geigy Corporation, Saw Mill River Road, Ardsley, N.Y., for oleoyl sarcosme. I

Example I, as tabulated below, is the preferred embodiment of the erasable writing medium previously described. I

EXAMPLE I COMPONENT PER CENT GANTREZ RESIN M556 28.0

Toluene 28.0

CYANBLUE BNF 55-3750 Pigment H3 Miscellaneous QUADROL 14.]

SARKOSYL 0 3.6

Mineral Oil 3.6

DOWANOL EPH l 1.4

The per cent of each of the functional components present may be varied within certain limits without adversely affecting the erasable characteristics of the writing medium described in Example I. These limits are: GANTREZ RESIN M556 from 25.0 to 30.0 per cent; toluene from 24.0 to 29.0 per cent; and CYANBLUE BNF 55-3750 from 9.0 to l3.0 per cent.

For each variation made in the relative quantity of a functional component, it may or may not prove necessary to make quantitative adjustments in the quantities of the various miscellaneous components; the nedd for and the nature of such adjustments should be readily determinable by those skilled in the art.

In the preparation of the erasable writing medium, it is desirable that the functional components be added in a specified order as may be readily seen from the following description of the preferred procedure. The most suitable mixing equipment should be equipped with heavy-duty bladesof either the sigmaor overlapping type and should be fitted with an easily removable but tight-fitting cover. It is also desirable that the bowl of the mixing equipment he of the tilting type to facilitate the removal of the erasable writing medium therefrom upon completion of the mixing operation.

Step 1 Add to the bowl of the mixer, with its blades turning, the desired quantity of the GANTREZ RESIN M556 dissolved in toluene. (This resin is marketed by the GAF Corporation in the form of a solution of resin, per cent solids, in toluene. This solution may be used in the as-received condition; additional toluene may be added thereto prior to be added to the mixer; or toluene may have been removed from the solution prior to being added to the mixer by heating the 50 per cent solids solution for a suitable length of time.)

Step 2 Add the desired quantity of CYANBLUE BNF -3750 in the form of a dispersion of the pigment in QUADROL with the blades turning; it is mandatory that this dispersion be added slowly and preferably in small increments. (This dispersion must have been prepared previously. The procedure used in preparing such a dispersion consists of mixing the desired quantities of pigment and QUADROL in a pony mixer, or equivalent, and then passing the paste so obtained successively through a 3-roll mill until the grind obtained is sufficiently fine to be rated as 8 or higher on a Hegemann Dispersion Gage.)

Step 3 Allow the mixer to run with the cover in place for a period of 2 hours following the completion of the addition of the dispersion.

Step 4 Add the remaining components of the formulation slowly and singly, in the following order with the blades of the mixer turning: DOWANOL EPH, SARKOSYL O," and mineral oil.

Step 5 Upon the completion of Step 4. allow the mixer to continue to run for 4 hours with the cover in place.

The erasable writing medium as disclosed herein is most suitable for use in a ball point writing instrument. Depending upon the relative quantity of pigment used, this writing medium may or may not flow with sufficient rapidity through the feed passageways of the conventional, gravity-fed ball point pen to provide a written line of adequate density. If it is found that gravity alone does not provide an adequate flow of the writing medium, the medium ofthis invention can be dispensed in a pressurized ball pen of the type disclosed in U.S. Pat. No. 3,099,252 that issued on July 30, 1963 to Eugene P. Cofield, Jr., a co-inventor of the present invention.

What is claimed is:

I. An erasable writing medium suitable for use in ball point pens, comprising a colorant and a matrix material capable of capturing said colorant having cohesive properties exceeding is properties to adhere to the solid members of the writing surface, said matrix material incorporated in a vehicle comprising a volitalizing component, said matrix material being a polyvinyl methyl ether having a viscosity in a 50% solid solution in toluene of 25,000 i 2000 cps at 77F, said volatilizing component being a solvent for said matrix material and having an evaporation rate of 3 to 15 on a relative numerical scale on which ethyl ether is assigned an evaporation rate of l, the amount of said colorant ranging from 9.0% to 13.0% by weight; and the amount of said volatilizing component ranging from 24.0% to 29.07: by weight, the amount of said matrix ranges from 25-30% by weight.

2. An erasable writing medium claimed in claim 1 wherein said other components include a pigment dispersing agent, a corrosion inhibitor, a lubricant, a viscosity adjustment solvent and a wettability adjustment agent.

3. An erasable writing medium as claimed in claim 1 wherein said colorant is a pigment-type material.

4. An erasable writing medium as claimed in claim 1 wherein said colorant is a copper phthalocyanine.

5. An erasable writing medium claimed in claim 1 wherein said volatizing component is toluene.

6. An erasable writing medium as claimed in claim 1 wherein said pigment dispersing agent and said corrosion inhibitor include NzNzN :N' tetrakis-(2-hydroxypropyl)-ethylenediamine.

7. An erasable writing medium as claimed in claim 1 wherein said lubricant is a white mineral oil.

8. An erasable writing medium as claimed in claim 1 wherein said viscosity adjustment solvent includes mixed phenyl ethers of ethylene glycol and diethylene glycol.

9. An erasable writing medium suitable for use in ball point pens, comprising a colorant and a matrix material capable of capturing said colorant having cohesive properties exceeding its properties to adhere to the solid members of the writing surface, said matrix material incorporated in a vehicle comprising a volatilizing component, said volatilizing component being a solvent for said matrix material and having an evaporation rate of 3 to 15 on a relative numerical scale on which ethyl ether is assigned an evaporation rate of l, and further comprising a sufficient amount of other components to produce a form suitable for use in a ball point writing instrument; and consisting of the following ingredients in the percentages by weight listed: polyvinyl methyl ether, 28% toluene, 28%; copper phthalocyanine pigment, 1 1.3%; NzNzN'zN tetrakis (2-hydroxypropyl) ethylencdiamine, l4.l; oleoyl sarcosine, 3.6%; mineral oil. 3.6%; and ll.4% of a mixture of phenyl ethylene glycol and diethylene glycol.

10. An erasable writing medium suitable for use in ball point pens, comprising a colorant and a matrix material capable of capturing said colorant having cohesive properties exceeding its properties to adhere to the solid members of the writing surface, said matrix material incorporated in a vehicle comprising a volitalizing component, said matrix material being a polyvinyl methyl ether having a viscosity in a 50% solid solution in toluene of 25,000 i 2000 cps at 77F, said volatilizing component being a solvent for said matrix material and having an evaporation rate of 3 to on a relative numerical scale on which ethyl ether is assigned an evaporation rate of l, the amount of said colorant ranging from 9.0% to 13.0% by weight; and the amount of said volatilizing component ranging from 24.0% to 29.0% by weight and further comprising a sufficient amount of other components to produce a form suitable for use in a ball point writing instrument, the named ingredients being compatible with each other and said matrix material being present in such an amount that when said medium is applied to writing paper, said matrix material will retard penetration of said colorant to a depth which would render erasure difficult.

11. An erasable writing medium as defined in claim 10 wherein said solvent is toluene.

12. An erasable writing medium as claim in claim l0 wherein the amount of said colorant ranges from 9.0% to 13.0% by weight; the amount of said matrix material ranges from 25.0% to 30.0% by weight and the amount of said volatilizing component ranges from 24.0% to 29.0%.

13. An erasable writing medium as defined in claim 10 wherein said colorant is a copper phthalocyanine.

=l= l l UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT N0. ,105

DATED April 1, 1975 INVENTOR(S) Phillip M. Daugherty, et' al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title Page, under "[75]" at line 3, after "Stockbridge" insert ,Ga. after "Tidd," insert Chamblee, 6a.; at line 4, before "Atlanta" insert Eugene P. Cofield, Jr., cancel "both of".

Signed and Scaled this Fourteenth Day of Aprill98l [SEAL] Anal:

RENE D. TEGTMEYER Aerating Oficer Acting Cmrmissiour of Patents and Trademark; I 

1. AN ERASABLE WRITING MEDIUM SIUTABLE FOR USE IN BALL POINT PENS, COMPRISING A COLORANT AND A MATRIX MATERIAL CAPABLE OF CAPTURING SAID COLORANT HAVING COHESIVE PROPERTIES EXCEEDING IS PROPERTIES TO ADHERE TO THE SOLID MEMBERS OF THE WRITING SURFACE, SAID MATRIX MATERIAL INCORPORATED IN A VEHICLE COMPRISING A VOLITALIZING COMPONENT, SAID MATRIX MATERIAL BEING POLYVINYL METHYL ETHER HAVING A VISCOSITY IN A 50% SOLID SOLUTION IN TOLUENE OF 25,000$2000 CPS AT 77*F, SAID VOLATILIZING COMPONENT BEING A SOLVENT FOR SAID MATRIX MATERIAL AND HAVING AN EVAPORATION RATE OF 3 TO 15 ON A RELATIVE NUMERICAL SCALE ON WHICH ETHYL ETHER IS ASSIGNED AN EVAPORATION RATE OF 1, THE AMOUNT OF SAID COLORANT RANGING FROM 9.0% TO 13.0% BY WEIGHT; AND THE AMOUNT OF SAID VOLATILIZING COMPONENT RANGING FROM 24.0% TO 29.0% BY WEIGHT, THE AMOUNT OF SAID MATRIX RANGES FROM 25-30% BY WEIGHT.
 2. An erasable writing medium as claimed in claim 1 wherein said other components include a pigment dispersing agent, a corrosion inhibitor, a lubricant, a viscosity adjustment solvent and a wettability adjustment agent.
 3. An erasable writing medium as claimed in claim 1 wherein said colorant is a pigment-type material.
 4. An erasable writing medium as claimed in claim 1 wherein said colorant is a copper phthalocyanine.
 5. An erasable writing medium claimed in claim 1 wherein said volatizing component is toluene.
 6. An erasable writing medium as claimed in claim 1 wherein said pigment dispersing agent and said corrosion inhibitor include N: N:N'' :N'' tetrakis-(2-hydroxypropyl)-ethylenediamine.
 7. An erasable writing medium as claimed in claim 1 wherein said lubricant is a white mineral oil.
 8. An erasable writing medium as claimed in claim 1 wherein said viscosity adjustment solvent includes mixed phenyl ethers of ethylene glycol and Diethylene glycol.
 9. An erasable writing medium suitable for use in ball point pens, comprising a colorant and a matrix material capable of capturing said colorant having cohesive properties exceeding its properties to adhere to the solid members of the writing surface, said matrix material incorporated in a vehicle comprising a volatilizing component, said volatilizing component being a solvent for said matrix material and having an evaporation rate of 3 to 15 on a relative numerical scale on which ethyl ether is assigned an evaporation rate of 1, and further comprising a sufficient amount of other components to produce a form suitable for use in a ball point writing instrument; and consisting of the following ingredients in the percentages by weight listed: polyvinyl methyl ether, 28% toluene, 28%; copper phthalocyanine pigment, 11.3%; N:N:N'':N'' tetrakis - (2-hydroxypropyl) -ethylenediamine, 14.1; oleoyl sarcosine, 3.6%; mineral oil, 3.6%; and 11.4% of a mixture of phenyl ethylene glycol and diethylene glycol.
 10. An erasable writing medium suitable for use in ball point pens, comprising a colorant and a matrix material capable of capturing said colorant having cohesive properties exceeding its properties to adhere to the solid members of the writing surface, said matrix material incorporated in a vehicle comprising a volitalizing component, said matrix material being a polyvinyl methyl ether having a viscosity in a 50% solid solution in toluene of 25,000 + or - 2000 cps at 77*F, said volatilizing component being a solvent for said matrix material and having an evaporation rate of 3 to 15 on a relative numerical scale on which ethyl ether is assigned an evaporation rate of 1, the amount of said colorant ranging from 9.0% to 13.0% by weight; and the amount of said volatilizing component ranging from 24.0% to 29.0% by weight and further comprising a sufficient amount of other components to produce a form suitable for use in a ball point writing instrument, the named ingredients being compatible with each other and said matrix material being present in such an amount that when said medium is applied to writing paper, said matrix material will retard penetration of said colorant to a depth which would render erasure difficult.
 11. An erasable writing medium as defined in claim 10 wherein said solvent is toluene.
 12. An erasable writing medium as claim in claim 10 wherein the amount of said colorant ranges from 9.0% to 13.0% by weight; the amount of said matrix material ranges from 25.0% to 30.0% by weight and the amount of said volatilizing component ranges from 24.0% to 29.0%.
 13. An erasable writing medium as defined in claim 10 wherein said colorant is a copper phthalocyanine. 